Nuclear power reactors are used in nuclear power stations supplying electrical power to a power transmission grid. The fuel assemblies that comprise the core of such reactors must be provided with coolant to remove thermal energy produced during the processes of nuclear fission and radioactive decay of fission by-products. Both such processes are on-going during power production operations. Direct-fission-produced thermal energy production is the principal heat generation process during reactor power generation operations. The radioactive decay process is on-going at not-insignificant thermal energy production rates even weeks or months following cessation of power generation operations.
Cooling of the fuel assemblies is accomplished during power production operations by circulating large total flow rates of reactor coolant through the fuel assemblies. Cooling is accomplished during reactor shutdown operation modes by maintaining the core well covered with liquid coolant that is undergoing heat removal via circulation at comparatively low total flow rates through the heat exchangers of a residual heat removal system.
For normal power generation operations, the thermodynamic efficiency of the power generation cycle of the nuclear power station is improved by operating the reactor (and consequently its coolant) at very high pressures and temperatures. For boiling water reactors (BWRs) used in such nuclear power stations, the reactor vessel housing the nuclear core contains coolant in the forms of slightly subcooled water, saturated water, steam-and-water mixture, and saturated steam--all at temperatures and pressures at or close to 546.degree. F. and 1020 psia, respectively.
Various postulated events may cause the reactor to experience a partial loss of its coolant inventory. A subcategory of such loss-of-coolant inventory events includes loss-of-coolant accident (LOCA) events, in which an hypothesized pipe break results in the reactor coolant inventory to be expelled from the reactor due to the initial high pressure and temperature of the coolant. Isolation valves are installed on lines connecting to the reactor to prevent or at least mitigate the extent of coolant inventory loss. However, in certain other hypothetical situations, such as for pipe breaks occurring between the reactor and the pipeline innermost isolation valves, the action of isolation valve closures could not prevent the reactor from undergoing a full blowdown. Additionally, event scenarios may include an intentional reactor full controlled depressurization because of the hypothesized nonfunctioning of high-pressure coolant injection systems and the consequent need to depressurize promptly so that low-pressure coolant injection systems can accomplish needed coolant resupply. To prevent serious core damage given such casualty scenarios, it is necessary to design the reactor to have enough initial coolant inventory to keep the nuclear core at all times covered with coolant. This prevents the fuel rods of the nuclear fuel assemblies from heating beyond acceptable levels.
Several emergency core cooling schemes have evolved to insure that the reactor core is properly cooled during a LOCA. The complement of safety grade systems that are provided as part of the nuclear steam supply system to meet these needs for adequate assured core cooling, are known as the emergency core cooling system (ECCS). For example, one BWR product line produced by GE Nuclear Energy uses both high-pressure as well as low-pressure injection of water as major elements comprising its ECCS. Considerable energy must be expended during the casualty to effect the required high-pressure injection of coolant. ECCS systems providing these high pressures must be brought on line while the casualty is occurring. In addition, these systems depend on the long term operation of power supplies such as emergency diesel generators and connected electrical pumps and are therefore expensive when designed to the required margins of reliability.
Advanced designs such as the simplified boiling water reactor (SBWR) seek to avoid reliance on pumping systems during a LOCA. These systems employ in new ways the large pool of water known as a "suppression pool" which is connected by pipes to the reactor. The suppression pool in the SBWR design is located within the reactor containment at an elevation higher than the core. The water in the suppression pool can now be used to flood the reactor core by gravity action alone after the reactor has depressurized following a LOCA.
The SBWR ECCS now consists of the aforesaid suppression pool, plus the aforesaid injection lines which connect the suppression pool to the reactor, plus a depressurization subsystem. Several limitations arise in connection with this conventional SBWR ECCS. First, the depressurization subsystem is required to reduce reactor pressure very rapidly. In addition, an adequate initial inventory of reactor coolant must be contained within the reactor vessel to counterbalance the coolant inventory lost because of flashing during depressurization. The initial water inventory must be such that the residual water inventory after flashing will keep the core covered by coolant until additional water is gravity injected by the suppression pool. As a result, the SBWR is required to have more initial water inventory than is needed by a conventional BWR. Approximately 15 to 20 feet of extra reactor vessel height is required to meet the needs for emergency core cooling for the SBWR.
For SBWR, this 15- to 20-foot region is also used as a chimney that promotes coolant circulation through the fuel assemblies. The two-phase steam/water mixture generated by heating water in the reactor core naturally up-flows through this region from the reactor core through standpipes to a steam/water separator assembly. Saturated liquid separated from the two-phase mixture by the steam separator assembly is discharged back into the reactor region external to the chimney. The discharged saturated water then flows at low velocities back into the reactor downcomer where it undergoes mixing with the cooler feedwater being returned to the reactor. The now-mixed coolant is at reactor pressure and is 20.degree. to 30.degree. subcooled, and so is still extremely hot. Because of its high temperature, a substanial fraction of this "hot" coolant will flash into steam during reactor depressurization following a LOCA.
For reactor coolant initially at 546.degree. F. and 1020 psia, depressurization of the reactor to 212.degree. F. and atmospheric pressure following a LOCA will result in approximately one-third of the water mass being flashed as steam and two-thirds remaining as water. The additional reactor vessel height and volume required to compensate for this coolant inventory lost due to flashing during depressurization of the reactor leads to a substantial increase in the capital cost of the nuclear island portion of the power station.